The Hubble Space telescope has inched forward our understanding of the mystery of dark matter. John Blakeslee and his team at the NRC Herzberg Astrophysics Program at the Dominion Astrophysical Observatory in Victoria, B.C., using Hubble's Advanced Camera for Surveys, has discovered the largest known grouping of "globular star clusters." According to NASA, globular star clusters are thick clusters of hundreds of thousands of stars, and include some of the oldest surviving stars in the universe. Almost 95 percent of globular cluster formation had occurred within the first 1 billion or 2 billion years after the universe was created in the big bang around 13.7 billion years ago. Astronomers estimate the population of stars at approximately 160,000 in the galaxy cluster known as Abell 1689.

According to last Thursday's Space Telescope Science Institute press release, the discovery is significant for two reasons. Studying globular clusters helps scientists understand when galaxies first formed during the first stages of concentrated star formation, with 95 percent of their formation occurring within the first 1 billion or 2 billion years after the universe was born 13.7 billion years ago. But the revelation that many astronomers and physicists have been waiting for is that the study of globular clusters may reveal more information about how much dark matter lurks in the gargantuan expanse of dark galaxy clusters such as the population of stars in question in Abell 1689. These oldest surviving stars of the universe "were very efficient in forming in the denser regions of dark matter near the center of the galaxy cluster" according to John Blakeslee, National Research Council Canada's Herzberg Institute of Astrophysics, shedding light on what he calls "regions of higher dark matter density."

Blakeslee's team's serendipitous discovery is an enormous group of such "star elders" that are twice as large as any other population found in previous globular cluster surveys. These Hubble observations have now documented the farthest such systems ever studied, at 2.25 billion light-years away. The research team's most significant finding is that globular star clusters are closely interrelated with dark matter. "In our study of Abell 1689, we show how the relationship between globular clusters and dark matter depends on the distance from the galaxy cluster's center," commented Karla Alamo-Martinez of the Center for Radio Astronomy and Astrophysics at Morelia's National Autonomous University of Mexico, and primary author of the paper. "In other words, if you know how many globular clusters are within a certain distance, we can give you an estimate of the amount of dark matter."

Scientists have approached, but have not refined a complete understanding of dark matter, as opposed to "baryonic matter," the 'regular' matter of galaxies, stars, planets, asteroids and organic life in Earth's biosphere that is composed of atoms. Dark matter does not emit light and does not absorb light or other forms of electromagnetic energy. But, scientists have observed dark matter's gravitational effects on other stellar objects. Caltech physicist Sean Carroll, in his blog, Preposterous Universe, writes, "Dark matter is some kind of particle that we have not yet detected in experiments here on Earth, but nevertheless comprises most of the matter in the universe. The first evidence for its existence came from studying the dynamics of galaxies and clusters of galaxies. The basic point is that something in orbit around a massive object moves more rapidly, the more mass the object has."

Carroll also explains that Zwicky first concluded that the motion of galaxies in the Coma cluster "were too rapid to be accounted for by the visible matter in the galaxies." Rubin later added weight to Zwicky's original observation by looking at matter orbiting at the edges of individual galaxies and observing a similar phenomenon, that "the rotation speeds of the galaxies did not fall off with distance as they should if the gravitational fields were being caused by the visible matter alone." These are two observations that warrant why the Space Telescope Science Institute calls dark matter "the underlying gravitational scaffolding upon which stars and galaxies are built." Furthering the study of dark matter may lead to a better understanding of how galaxies and galaxy clusters were formed billions of years ago, in the first breath of the cosmos.

According the the STSI press release, the newly announced study findings show that the majority of Abell 1689's globular clusters formed near the center of the galaxy cluster, with a high concentration of dark matter. The number of globular clusters decreased the farther away Hubble looked from its core area, and the amount of dark matter dropping equivalently.

Blakeslee said, "The globular clusters are fossils of the earliest star formation in Abell 1689, and our work shows they were very efficient in forming in the denser regions of dark matter near the center of the galaxy cluster." He added that, "Our findings are consistent with studies of globular clusters in other galaxy clusters, but extend our knowledge to regions of higher dark matter density."

Of significant interest is that most of the globular clusters observed have a brightness estimated at 31st magnitude, which is out of reach for the Hubble Space Telescope, but not for NASA's James Webb Space Telescope, coming on line later in the decade.

As Jack Horkheimer always said, "Keep Looking Up!"

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